Electrolysis cell current efficiency

ABSTRACT

A FUSED SALT ELECTROLYSIS CELL BATH IS SUBJECTED TO AT LEAST ONE THAW-FREEZE CYCLE, A THAW CONSISTING ESSENTIALLY OF MELTING THE CELL BATH BY HEATING THE BATH TO A TEMPERATURE ABOVE THE MELTING POINT OF SAID BATH, AND A FREEZE CONSISTING ESSENTIALLY OF SOLIDIFYING THE MELTED BATH BY COOLING SAID MELTED BATH TO A TEMPERATURE BELOW THE MELTING POINT OF THE BATH.

United States Patent O 3,558,451 ELECTROLYSIS CELL CURRENT EFFICIENCY Rudolf E. Svadleuak, Lewiston, N.Y., assignor to E. I. du Pont de Nernours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed Aug. 29, 1967, Ser. No. 663,977

Int. 'Cl. C22d 3/06 U.S. Cl. 204-68 1 Claim ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION In the operation of alkali metal fused salt cells, a fused salt mixture is electrolyzed to produce alkali metal at the cathode and halogen gas at the anode. The anode is conventionally a cylindrical graphite or carbon anode surrounded by an annular metallic cathode. A porous diaphragm is provided in the anode-cathode annular space to assist in the separation of the products of electrolysis.

Metallic sodium is produced generally from a molten mixture of the chlorides of calcium and sodium in electrolytic cells of the Downs type (US. Pat. No. 1,501,756) or in modifications of these cells. These cells are characterized by having one or more bottom mounted vertically aligned cylindrical graphite anodes each of which is projected upwardly into a separate cylindrical opening within a unitary cathode assembly. The sodium is produced at the cathode surfaces and the chlorine at the anode surfaces. Due to the difference in densities between these products, and that of the molten bath, the products rise to the surface of the bath and are then collected.

The aforementioned cells of prior practice have given good results, but unfortunately cells of this type rarely ever achieve current efficiencies greater than 80-85% (of. Sodium, A.C.S. monograph 133, p. 31 M. Sittig, Reinhold Publishing Corporation, 1956). Alternate salt mixtures for use as electrolyte baths have been described in the prior art and, although some of these mixtures have shown improved current efficiencies, this advantage has been offset by high material costs or production of sodium whose quality does not meet present high purity requirements and is not readily purified. Grabau, US. Patent 464,097 (Dec. 1, 1891) disclosed a ternary mixture consisting of sodium chloride, another alkali metal halide and alkaline earth halides. This mixture was stated to have a current efficiency of 95% but yielded sodium containing another alkali metal as an impurity. With the preferred other alkali metal salt, potassium chloride, the sodium was stated to contain 3% potassium. Seward et al., US. Patent 841,724 (Jan. 22, 1907) described a mixed salt bath containing sodium chloride, sodium fluoride and an alkaliearth chloride. This bath gave sodium free of other alkali metals but its current efficiency was not disclosed. However, neither of these baths appeared to have found commercial acceptance. More recently, Cathcart et al., US. Patent 2,850,442, have devised a mixture comprising sodium chloride,

barium chloride and strontium chloride. This electrolyte yields sodium containing less than 0.1% impurities and shows current efficiencies of 85-89% which can be raised 'ice to -95% by addition of 12% sodium fluoride. However, this mixture is expensive because of its strontium compound in that the original investment and maintenance are of a relatively high order. Wood, US. Patent 2,876,- 181, obtains high current efiiciency with a lithium chloride-sodium chloride electrolyte 'but the sodium is stated to contain 4% lithium under exemplary operating conditions and lithium electrolyte is expensive.

It is apparent that a tremendous effort has been exerted over the past years to achieve greater current efficiency. In view of the magnitude of the sodium industry coupled with the importance of sodium as a chemical intermediate and the rising power costs, the development of a relatively low cost, readily operable bath of improved current efiiciency is of outstanding industrial importance. Thus, a process for producing sodium having a relatively high degree of purity while operating at current efiiciencies ranging from 90-99% is highly desirable.

SUMMARY OF THE INVENTION This invention relates to a novel process for improving the current efficiency of an alkali metal fused salt electrolysis cell comprising subjecting the cell bath to at least one thaw-freeze cycle, a thaw consisting essentially of melting the cell bath by heating the bath to a temperature above the melting point of said bath, and the freeze consisting essentially of solidifying the melted bath by cooling said melted bath to a temperature below the melting point of the bath.

It has been discovered that this thaw-freeze process increases the current efliciency of a fused salt electrolysis cell in a cheap, efficient and commercially applicable manner.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The novel process of this invention for improving the current efiiciency of a used alkali metal fused salt electrolysis cell bath comprises subjecting the. solid salt bath of the cell to at least one thaw-freeze cycle. A thawfreeze cycle consists essentially of melting the solid cell bath by heating the bath to a temperature above the melting point of said bath and then re-solidifying the melted bath by cooling said melted bath to a temperature below the melting point of the bath. Thus, the term thaw designates melting the solid cell bath while the term freeze designates solidifying the melted or thawed cell bath. This invention resides in the unexpected discovery that after a cell bath undergoes a thaw-freeze cycle, its current efficiency is greatly increased.

In normal commercial operations, a sodium cell may be operated for a period of several months before the current efficiency significantly decreases. At this point a portion or all of the used cell bath is removed from the electrolysis cell. The cell bath is placed in a separate container and solidifies as the temperature decreases. Then the solidified cell bath is subjected to a thaw-freeze cycle. .For example, the cell bath may then be heated to a temperature of approximately 600 C. to melt the cell bath. After a few minutes, the cell bath is then subjected to the freeze cycle by cooling the bath below the melting point of the bath. It is this separate and distinct thawfreeze cycle which causes the current efliciency to increase. This rejuvenated cell bath may then be placed in an electrolysis cell and operated at an increased current efficiency.

The reasons for a current efliciency recovery from the thaw-freeze cycle are not fully understood. While this invention is not intended to be based on any particular theory, it is speculated that poor current efficiency may be due to the presence of small particulate electronic conductors in the bath which serve to transfer metallic sodium to the anolyte by a concentration cell mechanism. Upon undergoing a thaw-freeze cycle, these particulate electronic conductors grow to such a size that they rapidly settle out of the molten bath and are ineffective in transferring sodium to the anolyte where the sodium is forever lost.

The temperature at which the thawing or melting is carried out is not critical. Of course, the temperature must be sufficiently high enough to melt all of the initial components in the fused salt cell bath. For example, the temperature must be high enough to melt the chlorides of calcium and sodium (in a sodium cell) but the temperature does not have to be high enough to melt impurities, such as iron, in the cell bath. Generally, in the case of a sodium cell bath, the temperature of 600-650 C. will be used.

Similarly, the temperature of the freezing or solidification is not critical. The temperature must be lowered sufficiently so that all of the components of the cell bath become solidified. Generally, in the case of a sodium cell bath, the bath is cooled to a temperature within the range of 20l00 C.

The thaw-freeze cycle is generally not dependent upon any length of time. In the thaw portion of the thaw-freeze cycle, it is desirable to melt the cell bath and hold it at that temperature for a few minutes. While thawing the cell bath for a few seconds would be operable, the increased current efficiency in most cases would be very small. It is preferred to hold the thawing temperature for a period ranging from 1 minute to hours in order to produce a cell bath which will possess the desired current efficiency when utilized in an electrolysis cell. In theory at least there should be sufficient time to allow the particulate electronic conductors to grow to such a size and to settle out of the molten bath. The length of time of the freeze is merely a matter of convenience and can be adjusted to suit the particular commercial needs.

It has been observed that when a solid plant bath undergoes a thaw-freeze cycle, the solid plant bath is whiter in color than it was before undergoing the thaw-freeze cycle. Similarly, solid baths which turn blue on treating with untraviolet light do not turn blue after undergoing a thaw-freeze cycle. Generally speaking, there is a correlation between the color of the bath and the current efiiciency. A bath which is oif-white in normal daylight or blue in color after exposure to ultraviolet light has a lower current efficiency than cell baths which are white under the same conditions.

The following examples are illustrative of the practice of this invention. All electrolyte compositions are reported as percent by weight.

Example I A substantially conventional Downs cell was charged with an electrolyte containing about 26% sodium chloride, 24% calcium chloride and a remainder of barium chloride. This bath had a melting point of about 560 C. and was operated with a direct current for several months at an average temperature of about 605 C. This cell was equipped with a wire gauze diaphragm between anode and cathode and the spacing between these electrodes was 1.5 inches. Electrolysis was carried out at about 38,000 amps. and at a cell voltage of about 7 volts.

A portion of the cell bath was transferred to a laboratory compartmental cell Where the average current efficiency was found to be 87%. A portion of the cell bath from the above described Downs cell was transferred to a beaker where it solidified. This solidified cell bath was then subjected to a thaw-freeze cycle by heating the bath to a temperature of 605 C. for a period of approximately 3 minutes and then cooling the molten bath to approximately room temperature. The bath was transferred to a laboratory compartmental cell where the current efficiency was measured and found to be 97%. This 10% increase in current efiiciency is attributed to the thawfreeze cycle of this invention.

Several cell bath samples were taken from conventional Downs cells which were operating in a plant on a commercial scale. The cell baths contained similar electrolyte compositions as described above and had operated for various periods of times ranging from 2-12 months. The cell baths were subjected to a thaw-freeze treatment. In all cases the current efliciency was measured before and after the thaw-freeze cycle. The results are reported in the following table:

TABLE I.INCREASED CURRENT EFFICIENCY BY THAW-FREEZING Reaction Current Percent Example Treatment to U.V eflieiency change 2 None Turns blue- 84 O ne thaw-freeze cycle. No change 91 +7 3 None N.D 83 One thaw-freeze cycle. N.D 08 +15 4 None N.D- 87 One thaw'freeze ey N.D 98 +11 5 None N.D- 82 One thaw-freeze cyele N.D 97 +15 6 one Turns blue.. 86 One thaw-freeze cycle No change. 90 +4 Nora: N.D.=Not determined.

Since it is obvious that many changes and modifications can be made in the above-described details without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited to said details except as set forth in the appended claim.

I claim:

1. A process for improving the current efficiency of alkali metal fused salt electrolysis cell comprising removing at least a portion of the used alkali metal fused salt bath to a separate container, subjecting the removed bath to at least one freeze-thaw cycle, wherein a freeze consists essentially of solidifying the melted bath to a temperature below the melting point of the bath and a thaw consists essentially of melting the bath by heating the bath to a temperature above the melting point of said bath, and finally returning the regenerated bath to the electrolysis cell.

References Cited UNITED STATES PATENTS 3,020,221 2/1962 Loftus 204--68 3,119,766 1/1964 Thomas 20468 3,257,297 6/ 1966 Paterson et a1. 20468 JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner 

